This invention relates to a projection lens system used in projection television or the like.
In the projection television, red, green and blue pictures formed on CRTs (cathode ray tubes) are projected respectively through three projection lenses on the screen to form a large color picture thereon. In order to reduce the size of such a projection television system or to miniaturize the latter, it is essential to use the projection lens which is large both in picture angle and in aperture ratio and excellent in image forming performance.
A variety of projection lenses have been proposed in the art in order to satisfy the above-described requirement.
In one example of the conventional projection lenses, only glass lenses are employed because they can be manufactured with high accuracy to form a high quality projection lens. In another example, only plastic lenses are used in order to reduce the manufacturing cost and to have a large aperture ratio. In another example, glass lenses and plastic lenses with aspherical surfaces are employed in combination.
Furthermore, in a projection television system as disclosed by Japanese patent application (OPI) No. 175019/85, in order to compensate the curvature of field to thereby maintain the image forming performance high, the surface of the picture plate of the CRT is made aspherical.
However, the conventional projection lens system comprising glass lenses suffers from the following difficulties: The improvement of the image forming performance with the large aperture ratio maintained unchanged results in an increase of the manufacturing cost and greatly lowers the marginal characteristic. The projection lens made up of three plastic lens elements with aspherical surfaces is suitable for obtaining a large aperture ratio; however, it is still disadvantageous in that, with the coma aberration taken into consideration, it is difficult to improve the image forming performance. In addition, the plastic lens is disadvantageous in that the refractive index, the configuration and the focal length change with temperature, i.e., the image forming performance changes with temperature. In the projection television system described above in which the surface of the picture plate of the CRT is made aspherical, the CRT used is limited, and therefore the projection lens for the CRT thus limited cannot be used for other CRTs.